Sorption cooling processes typically employ some adsorbent disposed in a metal vessel and on a metal screen or surface which provides support for the adsorbent and permits the adsorbent to be placed in contact with the fluid stream containing the adsorbable component over the range of conditions necessary for the adsorption and desorption. The metal structures and physical arrangement of these devices has placed certain process limitations which restrict the amount of adsorbent which actually comes in contact with the fluid stream, or is accompanied by heat transfer inefficiencies inherent in the disposition of the adsorbent.
In the operation of sorption cooling systems, generally there are two or more solid beds containing a solid adsorbent. The solid adsorbent beds desorb refrigerant when heated and adsorb refrigerant vapor when cooled. In this manner the beds can be used to drive the refrigerant around a heat pump system to heat or cool another fluid such as a process stream or to provide space heating or cooling. In the heat pump system, commonly referred to as the heat pump loop, or a sorption refrigeration circuit, the refrigerant is desorbed from a first bed as it is heated to drive the refrigerant out of the first bed and the refrigerant vapor is conveyed to a condenser. In the condenser, the refrigerant vapor is cooled and condensed. The refrigerant condensate is then expanded to a lower pressure through an expansion valve and the low pressure condensate passes to an evaporator where the low pressure condensate is heat exchanged with the process stream or space to be conditioned to revaporize the condensate. When further heating no longer produces desorbed refrigerant from the first bed, the first bed is isolated and allowed to return to the adsorption conditions. When the adsorption conditions are established in the first bed, the refrigerant vapor from the evaporator is reintroduced to the first bed to complete the cycle. Generally two or more solid adsorbent beds are employed in a typical cycle wherein one bed is heated during the desorption stroke and the other bed is cooled during the adsorption stroke. The time for the completion of a fall cycle of adsorption and desorption is known as the "cycle time." The upper and lower temperatures will vary depending upon the selection of the refrigerant fluid and the adsorbent. Some thermodynamic processes for cooling and heating by adsorption of a refrigerating fluid on a solid adsorbent use zeolite and other sorption materials such as activated carbon and silica gel. U.S. Pat. No. 4,138,850 relates to a system for solar heat utilization employing a solid zeolite adsorbent mixed with a binder, pressed, and sintered into divider panels and hermetically sealed in containers. The U.S. Pat. No. 4,637,218 to Tchernev relates to a heat pump system using zeolites as the solid adsorbent and water as the refrigerant wherein the zeolite is sliced into bricks or pressed into a desired configuration to establish an hermetically sealed space and thereby set up the propagation of a temperature front, or thermal wave through the adsorbent bed. The bricks used in U.S. Pat. No. 4,637,218 are preferably not more than 10 mm in thickness. U.S. Pat. No. 5,477,705 discloses an apparatus for refrigeration employing a compartmentalized reactor and alternate circulation of hot and cold fluids to create a thermal wave which passes through the compartments containing a solid adsorbent to desorb and adsorb a refrigerant. U.S. Pat. No. 4,548,046 relates to an apparatus for cooling or heating by adsorption of a refrigerating fluid on a solid adsorbent. The operations employ a plurality of tubes provided with parallel radial fins, the spaces between which are filled or covered with solid adsorbent such as Zeolite 13X located on the outside of the tubes. U.S. Pat. No. 5,518,977 to Dunne et. al., which is hereby incorporated by reference, relates to sorption cooling devices which employ adsorbent coated surfaces to obtain a high cooling coefficient of performance.
U.S. Pat. No. 5,585,145 discloses a method for providing an adsorbent coating on a heat exchanger which comprises applying a flowable emulsion including a binder agent, water and a solid adsorbent material to the surface of the heat exchanger. The disclosure states that the binder can be an adhesive and that the thickness of the adsorbent coating can be dipped, painted or sprayed with a drying step comprising heating the layer at temperatures greater than 150.degree. C. in order to obtain a durable adsorbent coating structure.
All of the methods for coating assembled tube and plate heat exchangers to date suffer from the problem of uneven build up of the adsorbent coating. That is, in dipped, sprayed, or painted based coating methods, the emulsion or slurry comprising the adsorbent typically flows to a base or root point where the plate is joined to the tube, where the adsorbent layer is thicker than desired. For example, when finned tubes are coated by a dipping or spraying technique, the adsorbent slurry covers the fins and accumulates at the lowest point or where the fin and tube surfaces are joined. In some sorption cooling applications which employ a thermal wave approach, such as disclosed in U.S. Pat. No. 4,548,046 to Jones et al, the accumulation of adsorbent between the fined surface of the tube is desired to form an adsorbent/fin matrix. However, in sorption cooling processes where it is required to achieve a uniform temperature gradient in the adsorber or the generator of a sorption cooling system in order to operate with rapid cycles and achieve high efficiencies, better methods are sought to obtain a uniform adsorbent coating which does not suffer from a root buildup of adsorbent.
It is a still further object of this invention to provide an adsorbent module which is mechanically simpler to operate and is less costly to construct than fixed bed adsorbers and rotating desiccant wheels.
It is the object of the instant invention to provide an improved sorption cooling system for use in waste heat recovery, space heating, and air conditioning systems which is not limited by the regeneration efficiency of the adsorbent.